This video by K & Company, may help illustrate the whole idea.
So, as far as I can tell, it is similar to a scrapbook but a little......messier. This type of collecting and showcasing, may appeal to the crafter who does not have as much time or attention to neat and tidiness.
Essentially, you start with a pretty notebook, and then you just paste things or add things that you like. Photos, ticket stubs, menus, ribbon, washi tape, buttons, whatever. The concept is nothing new. I have one from when I was a kid. The only difference now is that you can find cute, "official smashbook supplies" everywhere you look, including this adorable line from Paper Source.
Here are a few of my favorite Smashbook finds on Pinterest.